FIG. 10 is a structural view of the conventional optical information recording/reproducing apparatus described on Pages 58 through 62--"LD Array Head For DRAW"--of "Micro-Optics News"(Vol. 3, No. 1, edited on Feb. 4, 1985) by Ito and Ohta.
Referring to FIG. 10, reference numeral 1 designates a semiconductor laser array emitting two light beams a recording beam L1 of high intensity of light and reproducing beam L2 of low intensity of light. Reference numeral 2 designates a collimator lens for collimating the parallel light beams L1 and L2 emitted from the semiconductor laser array 1, and 3 designates a beam shaping prism for correcting the distribution of optical strength of the respective collimated beams L1 and L2.
Reference numeral 4 designates a polarizing beam splitter, which is adapted to transmit therethrough the beams L1 and L2 to a beam shaping prism, toward an information recording medium (to be discussed below) and to reflect toward an error detecting system (to be discussed below) a recording reflected beam L1' and a reproducing reflected beam L2' from the information recording medium.
Reference numeral 5 designates a reflecting mirror, and 6 designates a 1/4 wavelength plate, which are disposed on an optical path at the transmission side of the polarizing beam splitter 4.
Reference numeral 7 designates an objective lens for focusing onto the beams L1 and L2 on the information recording medium the beams L1 and L2, said beams having already passed the reflecting mirror 5 and 1/4 wavelength plate 6 respectively. 8 designates an information recording medium comprising an optical disc rotatable around a rotary shaft 8a, and 9 designates an information track 9 formed concentrically or spirally in the information recording medium 8.
Reference numeral 10 designates a convex lens disposed on an optical path at the reflection side of the polarizing beam splitter 4, and 11 designates a spatial filter disposed at the focal point of convex lens 10, which cuts off the recording reflected beam L1' reflected from the information recording medium 8 and allows only the reproducing reflected beam L2' to transmit through the lens 11.
Reference numeral 12 designates a beam splitter for dividing the reproducing reflected beam L2' which has already passed the spatial filter 11. The beam is directed toward a tracking error detection system and toward a focusing error detection system (both to be discussed below). Reference numeral 13 designates a two-divided photodetector for receiving the reproducing reflected beam L2' which has passed the beam splitter 12. Reference numeral 14 designates a convex lens, 15 designates a knife edge, and 16 designates a two-divided photodetector for receiving the reproducing reflected beam L2' through the knife edge 15. All of the components 14, 15 and 16 are disposed on the optical path at the reflection side of the beam splitter 12.
Reference numeral 17 designates a differential amplifier which takes in a difference between the two signals output from the two-divided photodetector 13. Reference numeral 18 designates a tracking actuator for driving the objective lens 7 in the traversing direction (in the direction of the arrow T) with respect to the information track 9 on the basis of an output signal of the differential amplifier 17. Reference numeral 19 designates a differential amplifier which takes in a difference between the two signals output from the two-divided photodetector 16, and 20 designates a focusing actuator for driving the objective lens 7 in the vertical direction (in the direction of the arrow F) with respect to the surface of the information recording medium 8 on the basis of the output signal of the differential amplifier 19.
In addition, the two-divided photodetector 13 and differential amplifier 17 constitute the tracking error detection system for detecting a tracking error of the reproducing beam L2 irradiated to the information recording medium 8. The convex lens 14, knife edge 15, two-divided photodetector 16 and differential amplifier 19, constitute the focusing error detection system for detecting a focusing error of the reproducing beam L2.
FIG. 11 is an illustration showing the irradiation states of the respective light beams L1 and L2 with respect to the information track 9 in FIG. 10. Reference numerals P1 and P2 designate two light spots, in other words, a recording spot and a reproducing spot, formed of the condensed beams L1 and L2, the arrow D designates the traveling direction by rotation of the information recording medium 8, and 21 designates a pit formed on the information track 9 by the recording spot P1.
Next, explanation will be given on operation of the conventional optical information recording/reproducing apparatus shown in FIGS. 10 and 11.
The recording beam L1 and reproducing beam L2 emitted from the semiconductor laser array 1 are collimated by the collimator lens 2 to the parallel light beams and further formed by the beam shaping prism 3 into two light beams having nearly symmetrical intensity-distribution with respect to the optic axis rotationally.
Nextly, the recording beam L1 and reproducing beam L2 are incident on the objective lens 7 through the polarizing beam splitter 4, reflection mirror 5 and 1/4 wavelength plate 6 and focused on the information track 9 at the information recording medium 8 to generate the recording spot P1 and reproducing spot P2.
The recording spot P1 is advanced with respect to the rotation direction of the information recording medium 8 to form on the information track 9 the pits 21 which are modulated according to the contents of information, and the lagging reproducing spot P2 reproducing, at the same time, the content of information included in the recorded pit.
Continuously, the recording reflected beam L1' and reproducing reflected beam L2' reflected from the information recording medium 8 are again incident on the polarizing beam splitter 4 through the objective lens 7, 1/4 wavelength plate 6 and reflection mirror 5. The reflected beams L1' and L2', which reciprocate through the 1/4 wavelength plate 6 so as to rotate in the polarizing direction of 90.degree., are reflected at the polarizing beam splitter 4 and image-formed on the spatial filter 11 by the convex lens 10. In this case, only the reproducing reflected beam L2' passes through the spatial filter 11, is divided by the beam splitter 12, and is received by the two-divided photodetectors 13 and 16.
Accordingly, the tracking error signal is detected by the push-pull method using the two-divided photodetector 13, the focusing error signal being detected by the knife-edge method. The tracking error signal and focusing error signal thus obtained are amplified by the differential amplifiers 17 and 19 so as to drive the tracking actuator 18 and focusing actuator 20, respectively.
Also, the sum of output signals (not shown) of two-divided photodetector 13 is gained to detect the quantity of light of reproducing reflected beam L2' so as to reproduce the information signal recorded on the information track 9 at the information recording medium 8.
The conventional optical information recording/reproducing apparatus records and reproduces the information as mentioned above. It is generally known that when the push-pull method detects a tracking error signal, the tracking offset becomes larger.
FIG. 12 is a characteristic graph showing the relation between the follow-up quantity of the objective lens 7 with respect to the information track 9 and the tracking offset quantity, which is described in, for example, "Optical head for write-once disk with two perpendicular axes" of "Optical Memory Symposium" (in 1985, Pages 97 through 102). FIG. 12 shows that, when a track follow-up amount of the objective lens 7 is 100 .mu.m, the tracking offset is generated by only about 0.08 .mu.m. Usually, a tolerance of tracking offset is about 0.05 to 0.1 .mu.m so that it is seen that an offset value of 0.08 .mu.m is about the limit of the tolerance.
FIG. 13 is a characteristic graph showing the relation between the inclination of the information recording medium 8 and the tracking offset quantity, which is described in Pages 224 through 229 of, for example, "On-land Composite Pregroove Method for High Track Density Recording" (by Y. Tsunoda et al), SPIE, Vol. 695, 1986. In this case, it is shown that the information recording medium 8 inclines at an angle of 1.degree. to generate a tracking offset of 0.11 .mu.m to exceed the aforesaid tolerance.
It is well-known that, when a twin-spot method for taking a difference in the quantities of reflected light of the side spots is used instead of the push-pull method, the aforesaid problem of tracking offset is almost solved. The twin-spot method is adopted to most pickups for CD (compact disc).
The twin-spot method is described in, for example, "Principles of Optical Discsystems" [(by G. Bouwhuis et al), Adam Hilger Ltd., in 1985, Pages 71 to 72].
However, it is difficult to use the twin-spot method for an apparatus using the two light beams L1 and L2 as shown in FIG. 10.
The reason for the above is that to obtain the side spots for detecting tracking error, when a diffraction grating is disposed between the collimator lens 2 and the beam shaping prism 3 as shown in FIG. 10, six light spots P1 through P6 are created merely within several ten .mu.m on the information track 9 as shown in FIG. 14. In FIG. 14, reference numerals P3 and P4 designate side spots generated from the recording beam L1, P5 and P6 designate side spots generated from the reproducing beam L2, and the distance between the recording spot P1 and the reproducing spot P2 is about 20 to 30 .mu.m.
It is necessary in performing the tracking control of reproducing beam L2 by the twin-spot method to pick up the reflected light only from the side spots P5 and P6. However, as shown in FIG. 14, since intervals between the adjacent light spots P1 through P6 on the information track 9 are very small as about 10 .mu.m, it is extremely difficult to separate the reflected light only of the respective side spots P5 and P6 by use of the spatial filter 11 or the like. Also, there is a great possibility that the recording reflected beam L1' is larger in strength than the reproducing reflected beam L2' and thus, may leak into the reflected lights of side spots P5 and P6, whereby it is more difficult to pick up the reflected light of side spots P5 and P6.
Also, it is difficult for an optical information recording/reproducing apparatus using one light beam for both the recording and reproducing information to adopt the twin-spot method. In this case, for example, during the information recording, the light spots formed on the information recording medium 8 are the recording spot P1 and side spots P3 and P4 (refer to FIG. 14) only, at which time the side spot P3 is positioned on the non-recording information track 9 and the side spot P4 is positioned on the already recorded information track 9.
Accordingly, even in the state where no tracking offset exists in the recording spot P1 during the information recording, the quantities of reflected lights of side spots P3 and P4 are different from each other, whereby the use of the twin-spot method results in the occurrence of tracking offset.
Furthermore, the relative-positional relationship of the recording spot P1 is mechanically adjusted to coincide with the position of the reproducing spot P2. But in the conventional apparatus in FIG. 10, despite that one objective lens 7 focuses the recording beam L1 and reproducing beam L2 on the information recording medium 8, the focusing error signal and tracking error signal are obtained only from the reflected light of the reproducing spot P2. Accordingly, when the information recording medium 8 becomes eccentric to even slightly deteriorate the parallelism between the line connecting two light spots P1 and P2 and the information track 9, the tracking offset is generated at the recording spot P1.
The conventional optical information recording/reproducing apparatus, as above-mentioned, detects the tracking error signal by use of the push-pull method, thereby creating the problem in that the tracking offset generated by the track follow-up of objective lens 7 or the inclination of information recording medium 8 becomes larger and the tracking control cannot be stabilized.
Also, since the focusing error signal and tracking error signal are obtained only from the reflected light of the reproducing spot P2, the parallelism between the line connecting the light spots P1 and P2 and the information track 9, deteriorates thereby making stable tracking control impossible.